I just checked that link for the SG90 and they say:
The TP SG90 is similar in size and weight to the Hitec HS-55, and is a good choice for most Parkflyers
This would lead me to believe that my existing Hitec 55 would actually fit like you have done, but I fail to see how, the mounting arms on the servo don't seem to overlap the servo tray enough to make holes to mount it with screws.
Sorry for double post.